How to load external resources using a reference URI in Java XML
Total stab in the dark (you really need to paste stacktraces...), but here goes.
I took a quick look at the source of ResourceResolver at:http://java2s.com/Open-Source/Java-Document/XML/xml-security-1.4.3/org/apache/xml/security/utils/resolver/ResourceResolver.java.htm
I can see there is going to be a NullPointerException due to a non initialised _resolverVector field in the ResourceResolver, unless the static init() method has already been called on ResourceResolver. Above the init() method, there is a comment:
The init() function is called by org.apache.xml.security.Init.init()The ResourceResolver itself does not appear to trigger that call - do you do it ? If not you should try it, prior to using any of the org.apache.xml.security stuff.
Firstly you need to understand what exactly is causing the NullPointerException so I would suggest splitting this code out so that you can see where the null value is.